I. Field
The present invention relates to data communication. More particularly, the present invention relates to harmonizing vocoder operations between non-compatible communication systems.
II. Description of the Related Art
The field of wireless communications has many applications including, e.g., cordless telephones, paging, wireless local loops, personal digital assistants (PDAs), Internet telephony, and satellite communication systems. A particularly important application is cellular telephone systems for remote subscribers. As used herein, the term “cellular” system encompasses systems using either cellular or personal communications services (PCS) frequencies. Various over-the-air interfaces have been developed for such cellular telephone systems including, e.g., frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). In connection therewith, various domestic and international standards have been established including, e.g., Advanced Mobile Phone Service (AMPS), Global System for Mobile (GSM), and Interim Standard 95 (IS-95). IS-95 and its derivatives, IS-95A, IS-95B, ANSI J-STD-008 (often referred to collectively herein as IS-95), and proposed high-data-rate systems are promulgated by the Telecommunication Industry Association (TIA) and other well-known standards bodies.
Cellular telephone systems configured in accordance with the use of the IS-95 standard employ CDMA signal processing techniques to provide highly efficient and robust cellular telephone service. Exemplary cellular telephone systems configured substantially in accordance with the use of the IS-95 standard are described in U.S. Pat. Nos. 5,103,459 and 4,901,307, which are assigned to the assignee of the present invention and incorporated by reference herein. An exemplary system utilizing CDMA techniques is the cdma2000 ITU-R Radio Transmission Technology (RTT) Candidate Submission (referred to herein as cdma2000), issued by the TIA. The standard for cdma2000 is given in the draft versions of IS-2000 and has been approved by the TIA. Another CDMA standard is the W-CDMA standard, as embodied in 3rd Generation Partnership Project “3GPP”, Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214.
Each standard defines how various types of information are processed for transmission. In a typical communication system, an encoder generates a stream of information bits representing voice or data traffic. This stream of bits is subdivided and grouped, concatenated with various control bits, and packed into a suitable format for transmission. Voice and data traffic can be transmitted in various formats according to the appropriate communication standard, such as, e.g., frames, packets, and subpackets. For illustrative ease, the term “frame” will be used herein to describe the transmission format in which traffic is carried over the transmission medium. However, the term “frame” will also be used herein to describe the output of a speech coder. The definition of the word will depend upon the context in which the word is used
A speech coder is a device that extracts parameters relating to a model of human speech generation and then uses these parameters to compress the speech for transmissions. Speech coders typically comprise an encoder and a decoder. A speech coder divides the incoming speech signal into blocks of time, or analysis frames. The encoder analyzes the incoming speech frame to extract certain relevant parameters, and then quantizes the parameters into binary representation. The binary representation is packed into transmission frames and is transmitted over a communication channel to a receiver with a decoder. The decoder processes the transmission frames, unquantizes them to produce the parameters, and resynthesizes the speech frames using the unquantized parameters. Speech coders are also referred to as voice coders, or “vocoders,” and the terms will be used interchangeably herein.
The function of the speech coder is to compress the digitized speech signal into a low-bit-rate signal by removing all of the natural redundancies that are inherent in speech. The digital compression is achieved by representing the input speech frame with a set of parameters and employing quantization to represent the parameters with a set of bits. If the input speech frame has a number of bits Ni and the output frame produced by the speech coder has a number of bits No, then the compression factor achieved by the speech coder is Cr=Ni/No. The challenge is to retain the high voice quality of the decoded speech while achieving a target compression factor. The performance of a speech coder depends on how well the speech model, or the combination of the analysis and synthesis process described above, performs, and how well the parameter quantization process is performed at the target bit rate of No bits per frame. Thus, the goal of the speech model is to capture the essence of the speech signal, or the target voice quality, with a small set of parameters for each frame.
Different types of speech coders are deployed in the various existing wireless communication systems, often using quite dissimilar speech compression techniques. Moreover, the transmission frame formats and processing that are defined by one particular standard is most likely different from those of other standards. For example, CDMA standards support the use of variable-rate vocoder frames in a spread spectrum environment while GSM standards support the use of fixed-rate vocoder frames and multi-rate vocoder frames. Similarly, Universal Mobile Telecommunications Systems (UMTS) standards also support fixed-rate and multi-rate vocoders, but not variable-rate vocoders. For compatibility and interoperability between these non-compatible communication systems, it is highly desirable to enable the support of variable-rate vocoder frames within GSM and UMTS systems, and the support of non-variable rate vocoder frames within CDMA systems. An example of a variable-rate vocoder is the Selectable Mode Vocoder (SMV), which is promulgated in IS-893; an example of a multi-rate vocoder is the Adaptive Multi-Rate (AMR) vocoder, which is promulgated in “ETSI EN 301 704 Digital Cellular Telecommunications System; Adaptive Multi-Rate (AMR) Speech Transcoding” (the AMR standard); and an example of a fixed-rate vocoder is a Enhanced Full Rate vocoder, which is promulgated in 3GPP TS 46.060: “Digital cellular telecommunications system (Phase 2+); Enhanced Full Rate (EFR) speech transcoding.”
One significant reason for promoting compatibility and interoperability between non-compatible systems is to enable the use of wideband vocoders between non-compatible systems. A “wideband” vocoder is one that codes speech within a frequency range of 7000 Hz. In a traditional landline telephone system, the transmission medium and terminals are bandlimited to 4000 Hz, so speech is typically transmitted in a narrow range of 300 Hz to 3400 Hz, with control and signaling overhead carried outside this range.
In view of the physical constraints of landline telephone systems, signal propagation within cellular telephone systems is implemented with these same narrowband frequency constraints so that calls originating from a cellular subscriber unit can be transmitted to a landline unit. However, cellular telephone systems are capable of transmitting signals with wider frequency ranges, since the physical limitations requiring a narrow frequency range are not present within the cellular system. An exemplary standard for generating signals with a wider frequency range is promulgated in document G.722 ITU-T, entitled “7 kHz Audio-Coding within 64 kBits/s,” published in 1989. Accordingly, wideband counterparts of the variable-rate and multi-rate vocoders recited above, have been developed. The wideband counterparts provide superior acoustical benefits over the narrowband vocoders.
When wideband signals are exchanged between two wideband terminals operating within a cellular system, additional processing and constraints must be imposed because the wideband signals are too “fat” for the narrowband transmission channel. Currently, the maximum data capacity for a public switched telephone network (PSTN) is 64 kbps. For a narrowband signal, 8000 samples/second must be obtained for accurate reconstruction of the original signal. Standard pulse code modulation (PCM) sample data is represented using 8-bit symbols. By using 8-bit symbols, the maximum data capacity of the PSTN connection is reached (8000 samples/sec×8 bits/sample=64,000 bps) while minimizing quantization errors. However, for a wideband signal, 16,000 samples/second must be obtained for accurate reconstruction of the original signal. Hence, the wideband signal is too “fat” for the narrowband transmission channel.
The problems arising from the physical constraints of the 64 kbps PSTN connection can be avoided by implementing tandem-free operations (TFO) between infrastructure entities in the network. Tandem-free operations refer to the bypass of vocoders within infrastructure entities in the network. When a tandem-free operation is implemented, a wideband signal from one terminal of the network can be conveyed over the PSTN to another terminal in the same network through the use of punctured 8-bit PCM symbols, wherein vocoder output bits are punctured into the PCM symbols.
In order to implement tandem-free operations, the vocoders at the transmission end and the receiving end must be compatible. This is not a problem when wideband signals are exchanged between terminals within the same communication network. Co-pending U.S. patent application Ser. No. 09/811,056 entitled “COMMUNICATIONS USING WIDEBAND TERMINALS,” filed on Mar. 15, 2001, now U.S. Pat. No. 7,289,461, issued on Oct. 30, 2007, addresses this problem. However, there is a problem when one wishes to exchange wideband signals between terminals of non-compatible networks.
For example, in a multiple access system such as CDMA, variable-rate vocoders are implemented. An example of a variable-rate vocoder is the Wideband Selectable Mode Vocoder (WB-SMV). However, in a multiple access system such as GSM, fixed-rate or multi-rate vocoders are implemented. An example of a multi-rate vocoder is the Wideband Adaptive Multi-Rate Vocoder (AMR-WB). Although the vocoder types are structurally and functionally different, it should be noted that common, generic terminology is shared between the vocoder types. For example, a “mode” in an AMR-WB vocoder refers to a vocoder frame with a fixed data rate. However, a “mode” in a WB-SMV vocoder refers to an average data rate, which is achieved by a mixture of different frame types. The meaning of the word should be read in context with the usage of the word. In order to minimize the confusion that might arise from using such commonly shared terms between the numerous types of vocoders, the embodiments that will be described below will use the WB-SMV vocoder configurations and terminology to represent variable-rate vocoders and the AMR-WB vocoder configurations and terminology to represent fixed-rate and multi-rate vocoders, rather than the narrowband versions. However, it should be understood that the configuration details could be extended to suit other vocoders without undue experimentation. The technical specification for the AMR-WB frame structure is found in the document 3GPP TS 26.201 V5.0.0 (2001-03). The technical specification for the WB-SMV frame structure is yet to be released.
Accordingly, the embodiments that are described below are for harmonizing the transmission of wideband signals between different vocoders of non-compatible systems, so that the acoustical benefits of wideband vocoders need not be sacrificed in transmissions between non-compatible systems.